Quantifying semi-volatile organic contaminants in solution by internal standard addition method requires prompt addition of the internal standards

The precision and accuracy of quantifying semi-volatile organic compounds (SVOCs) in solution by GC/MS, particularly when volume errors are unpredictable or difficult to control, are improved by utilizing internal standards (IS). Not obvious though is the extent to which timing IS addition affects measurement. To illustrate this fact, the mean concentrations of 60 SVOCs (40 or 80 mu g/mL) in two identical solutions into which IS were added at different times are compared in this study. In one solution, IS were added promptly on preparation (reference); in the other, IS were added after 36 days of incubation (treatment). To investigate the role that temperature might play here as well, equal fractions of each solution were incubated at -20 degrees C, 4 degrees C or 22 degrees C. Results, as determined by one-way ANOVA, show that there were no differences between the reference solutions at the beginning and after 36 days (F-3,F-236=0.244, p=0.865), but that significant differences exist between the reference solutions collectively and the treatment irrespective of temperature (F-6,F-413=6.76, p=1.99e(-06)). These results, confirmed by a post hoc analysis, suggest that uncertainty is introduced into SVOC quantitation when internal standards are not added promptly into SVOCs solutions on preparation.